1. Field of the Invention
The present invention relates to track access control of disk drives for moving a head to a target track depending on an access instruction from a host controller, and more particularly, to track access control methods and apparatus for optical disk drives for moving a beam emitted from an optical head to the target track through servo control of a tracking actuator.
2. Description of the Related Art
An optical disk drive has large storing capacity since the track pitch can be set in the order of several microns. For this and other reasons, there is interest in using such drives as a large capacity storage apparatus of a computer system.
The track access control system of an optical disk drives moves the beam emitted from an optical head to a target track designated by a read instruction or write instruction issued from a host controller. The track access control system controls the beam position and moving speed.
FIG. 1 is a diagram for explaining a conventional apparatus. In FIG. 1, an optical head 12 is provided for freely determining the position of the beam in the radial direction with a head drive motor 26 for an optical disk 10 which may be rotated at a constant speed, for example, of 3600 rpm by a spindle motor 24. A read/write operation of information is carried out to or from the optical disk 10 through irradiation of an optical beam from the optical head 12.
Within the optical head 12, a laser diode 28 is provided as a light source. The light beam from the laser diode 28 is guided to an objective 36 through a collimeter lens 30, a polarization beam splitter 32, and a .lambda./4 (1/4 wavelength) plate 34. The light beam is then narrowed to a beam spot by the objective 36 to irradiate an optical disk 10. The light beam reflected from the optical disk 10 is reflected in the right angle direction by the polarization beam splitter 32 and enters a 4-splitted photodetector 40 through a condenser lens 38.
In such an optical disk apparatus, many tracks are formed with a track pitch of 1.6 .mu.m, for example, in the case of ISO standard, in the radial direction of the optical disk 10. Therefore, only a little eccentricity results in a large deviation of track position. Moreover, waving of the optical disk results in deviation of the focal point of the beam spot. Accordingly, a beam spot of 1 .mu.m or less is required to follow such positional deviation.
For this reason, a focusing actuator 42 for adjusting the focal point by vertically moving the objective 36 of the optical head 12, and a tracking actuator 14 for causing the beam to follow the track by moving the objective 36 in the direction crossing the track, are provided.
The focusing actuator 42 is controlled by a focusing servo circuit 46. Namely, the focusing servo circuit 46 drives the focusing actuator 42 so that a focus error signal FES obtained from the receiving signal of the 4-splitted photodetector 40 is minimized.
The tracking actuator 14 is controlled by the tracking servo circuit 48 during track servo operation for positioning the beam to the target track, and is speed-controlled by a speed control circuit 50 during the track access for moving the beam to the target track for the next new access.
FIG. 2 is a diagram for explaining the conventional track access control. The speed control is carried out for feedback control of the tracking actuator so that speed error Ve between the target speed Vt and beam moving speed V is minimized in order to move the beam to the target track position from the initial track position S. Simultaneously, the track actuator is brought quickly to the target speed through acceleration control by applying an acceleration pulse voltage of +Va for a constant period at the time of initiating track access, while the tracking actuator is decelerated, at the time of completing the track access, by applying a deceleration pulse voltage of -Va for a constant period. Thereby, the tracking actuator reaches the target track while the beam speed is set to zero and initiates the position control (fine control).
The beam moving speed required for track access control can be detected theoretically for each 1/2 track from the zero cross period of the tracking error signal TES or gradient of signal (differential value) at the time of zero cross. However, in the actual apparatus, it is impossible to detect beam speed in every other 1/2 track but in every other track in order to eliminate the influence of DC offset of the tracking error signal TES.
However, for detection of the beam moving speed in such conventional track access control, when the beam moving speed is high, the zero cross interval of the tracking error signal TES becomes small, the quantity of information obtained in the unit time increases and stable beam position control can be realized. But, when the beam moving speed becomes low, the quantity of information obtained in each unit of time decreases and phase delay of the beam position control becomes significant. Therefore, accurate beam position control cannot be realized when the beam speed is low, which occurs immediately before completion of access to the target track. When the beam reaches the target track and changes the speed control to the position control (fine control), there is a problem in that the naturally unstable positioning to the target track becomes further unstable, and a longer time is required before the beam is positioned to the target track and read or write operations can be initiated.